Method of producing a molybdenum-steel slide surface on a light metal alloy

ABSTRACT

A method for producing cylinder linings in an aluminum alloy engine block for an internal combustion engine utilizes plasma spraying of a mixture of powdered molybdenum and powdered steel containing from about 10 to about 70 wt. % molybdenum and from about 90 to about 30 wt. % steel directly to the engine block. The resulting slide layer has good adhesion to the engine block and has high wear resistance.

REFERENCE TO RELATED APPLICATION

This application is a continuation of Application No. PCT/EP96/04745with an international filing date of Oct. 31, 1996.

BACKGROUND OF THE INVENTION

This invention relates to methods for producing a slide surface on alight metal alloy and also to reciprocating piston engines such asinternal combustion engines having cylinder linings with slide surfaces.

German Offenlegungsschrift No. 44 40 713 discloses a method of producingslide surfaces on cast iron parts consisting of several steps in whichlubrication pockets are exposed in a slide surface so as to providehydrodynamic lubrication in operation. These steps include machining ofthe slide surfaces and then processing the surfaces with a chemicallyand electrochemically inactive liquid applied under a pressure suitablefor flake removal. By a combination of liquid erosion and frictionalsmoothing the lubrication pockets are exposed in the slide surface,forming in the aggregate a system of pressure microchambers to providethe requisite hydrodynamic lubrication. These pockets are produced by ahoning operation which extracts small titanium carbide and nitrideparticles from the surface and the craters so formed are closed byfurther treatment. Liquid erosion and frictional smoothing expose thesepockets again.

U.S. Pat. No. 5,080,056 discloses a process in which a substantiallypore-free aluminum bronze alloy layer is applied by high velocity flamespraying to parts made of a an aluminum alloy, the thickness of theapplied alloy layer being then reduced by honing from about 1 mm to afinal dimension of about 127 microns.

In contradistinction to the formation of a system of pressuremicrochambers, it is known that a communicating groove system may beobtained, for example on the surfaces of cylinder liners for internalcombustion engines by a honing operation. Such honing producesintersecting scores which are interconnected at the points ofintersection and constitute an overall open system. A disadvantage ofthis system is that the part sliding on the slide surface, for example apiston ring on a piston of an internal combustion engine, will push oilcontained in the scores ahead of it, preventing any significant build-upof hydrodynamic oil pressure on the slide surface. Consequently, alongthe edges of the scores there will be mixed friction between the twomaterials. The mixed friction lubrication systems which are inwidespread use for surfaces of gray iron castings, however, are notapplicable to slide surfaces on parts made of an aluminum alloy forreasons of weight.

U.S. Pat. No. 2,588,422 discloses an aluminum engine block havingcylinder liners which are formed by thermal spraying. These liners arebuilt up in two layers on the untreated surface of the engine block, thetop layer being a hard slide layer such as steel about 1 mm in thicknessand the lower layer being a molybdenous interlayer about 50 microns inthickness. The interlayer, containing at least 60% molybdenum, does notconstitute a slide layer, but is necessary in order to bind the hardslide layer to the aluminum block. Preferably, the interlayer is made upof pure molybdenum. The slide layer is a layer of hard metal, as forexample carbon steel, bronze or stainless steel, in which the steel maybe an alloy containing nickel, chromium, vanadium or molybdenum forexample. In principle, this two-layer structure provides a good slidelayer, but the cost of the double coating is substantial.

British Published Patent Application No. 2,050,434 discloses variouscoatings obtained by thermal spraying and having thicknesses from 0.5 to2 mm. These coatings are formed on steel or cast parts for internalcombustion engines, as for example piston rings or cylinder linings. Inthis case it is found that coatings consisting of equal parts ofpowdered molybdenum and powdered carbon steel are considerably lessabrasion-resistant than coatings containing only 0.5-4.5 wt. % ofmolybdenum with 20-97 wt. % of metal carbides and possibly iron orferrous alloys. To bind these coatings to aluminum alloys, the processdescribed above with respect to U.S. Pat. No. 2,588,422 must be used.

British Patent No. 1,478,287 describes a powdered mixture for plasmacoating of steel parts or castings such as piston rings, cylinder blocksor cylinder linings to produce layers having a thickness of about762-1270 microns. The powder is a mixture of molybdenum, boron and castiron containing at least as much cast iron as molybdenum and the boronis usually present in amounts up to 3% of the sum of molybdenum and castiron. Such coatings, as example 1 of British Published PatentApplication No. 2,050,434A shows, no longer meet present-day performancerequirements.

Another coating for cylinder liners is disclosed in U.S. Pat. No.3,620,137 in which a plasma coating containing nickel and chromium,boron, silicon and possibly also iron in addition to at least 65 wt. %molybdenum is described. This coating is intended for cylinder linersmade of gray cast iron and exhibits very small pores of 0.1 to 2 micronsand an over-all porosity of 15%, corresponding to the coatings describedin British Application No. 2,050,434 and British Patent No. 1,478,287.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for producing a slide surface on a light metal alloy whichovercomes disadvantages of the prior art.

Another object of the invention is to provide a method of producing aslide surface on an aluminum alloy in which a single coating operationis sufficient.

A further object of the invention is to provide a reciprocating pistonengine having an engine block made of an aluminum alloy with improvedslide surfaces.

These and other objects of the inventors attained in a comparativelysimple manner by applying a wear layer to a metal part, by thermalspraying as described for example in German Standard DIN 32530, October1989, and thereafter machining the surface of the part to expose amicrochamber system providing lubrication pockets. In this way, a slidesurface is produced on a metal part which has a high wear resistance notprovided by the base material. The thermal spraying and ensuingmachining produce numerous eruptions in the surface of the materialwhich are healed only slightly during the machining, consequentlyproviding an adequate number of depressions to form the microchambersystem.

The pressure microchamber system on the slide surface is thus formed inan entirely different manner from the aforementioned prior art whichnecessarily relies upon the presence of titanium components.

In a preferred embodiment of the invention, the wear layer is applied byplasma spraying, in particular atmospheric plasma sprayings.Plasma-sprayed layers are in themselves microporous to begin with sothat, using such layers, a single processing operation, for examplehoning, can produce a system of pressure microchambers in the coatingsurface.

The high-velocity flame spraying proposed in U.S. Pat. No. 5,080,056cannot be used in many cases, for example in the production of cylinderliners for internal combustion engines, because the comparatively highenergy input could cause the cylinder liners to warp.

The method according to the invention advantageously avoids anyenvironmentally contaminating galvanic step. By use of suitable metalpowders for plasma spraying and suitable honing procedures, extremelysmooth slide surfaces can be created with extremely low roughnessdepths, producing the desired pressure microchamber system forhydrodynamic lubrication.

According to the present invention, it has been found that a mixture ofpowdered steel with powdered molybdenum is especially suitable forthermal spraying to obtain slide surfaces on aluminum alloys. Thismixture preferably consists of about 10-70 wt. % powdered molybdenum andabout 90-30 wt. % powdered steel, with about 30-50 wt. % powderedmolybdenum and about 70-50 wt. % powdered steel being especiallypreferred. If desired, the mixture may be supplemented with additionalcomponents, preferably constituting less than 50 wt. % of the totalmixture. One of the most favorable mixtures has been found to be a 50:50mixture of steel and molybdenum powders. Even though such mixturescoated on cast iron substrates in the prior art lead only to moderatelyhard, i.e. not wear-resistant, coatings, as described in British PatentNo. 2,050,434, such a coating is extraordinarily adherent to light metalalloys, and is extraordinarily tight and abrasion resistant when coatedon aluminum. Thus, according to the invention, the slide surface isformed on a layer deposited directly on the aluminum alloy, requiring nointerlayer as is necessary according to U.S. Pat. No. 2,588,422. Thus, aunique feature of the present invention is that a wear-resistant layercan be applied with direct bonding to the comparatively soft substrateof an aluminum. Another improvement provided by the invention is thecoating of cylinders in aluminum engine blocks of a reciprocating pistonengine to produce wear-resistant slide surfaces using the method.

In thermal spraying of coating layers according to the invention, aplasma spray is preferably employed, and the mixture of metal to besprayed is preferably in the form of a powder. Since the powder sprayedonto the surface of the part is at least incipiently molten duringspraying, maintenance of a certain particle size distribution isadvantageous to the quality of the resulting coating. The use ofparticles having a size between about 2 and 70 microns, and preferablybetween about 5 and 50 microns, is especially advantageous. Preferablyabout 90 percent of the particles should have a size satisfying theforegoing requirements.

For use in preparing slide coatings according to the present invention,certain steels have proved especially advantageous. A number ofproperties of the steel, separately or combined, lead to especiallyfavorable slide coatings. Among these special properties is the hardnessof the steel for which various criteria may be critical. Thus, thehardness of the annealed steel, of the thermally sprayed steel, or elsethe hardness of the sprayed molybdenum-steel mixture, may be taken ascriterion. For the mixtures, another consideration as a criterion forthe steel selection is the hardness distribution throughout the mixture.Typical parameters for the hardness of the steel are as follows:

The thermally sprayed steel has a hardness from about 300 to about 500HV 0.3, preferably about 350 to about 450 HV 0.3;

The steel (annealed) before thermal spraying has a hardness from about300 to about 450 HV 0.3;

The steel has a hardness selected so that a thermally sprayed mixture of30 wt. % molybdenum or molybdenum alloy with up to about 40 wt. % ofother constituents and 70 wt. % of the steel has a hardness from about550 to about 850 HV 0.05, preferably from about 600 to about 750 HV0.05, desirably from about 650 to about 700 HV 0.05;

The steel has a hardness selected so that a thermally sprayed mixture of50 wt. % molybdenum or molybdenum alloy with up to 40 wt. % of otherconstituents and 50 wt. % of the steel has a hardness from about 550 toabout 850 HV 0.05, preferably from about 600 to about 750 HV 0.05,desirably from about 650 to about 700 HV 0.05;

The steel has a hardness selected so that thermally sprayed mixtures inthe range 5 from 30 wt. % molybdenum or molybdenum alloy with up to 40wt. % of other constituents/70 wt. % of the steel to 50 wt. % molybdenumor molybdenum alloy with up to 40 wt. % of other constituents/50 wt. %of the steel have a hardness in the range from about 550 to about 850 HV0.05, preferably from about 600 to about 750 HV 0.05, and desirably fromabout 650 to about 700 HV 0.05 a hardness difference of at most 100 HV0.05, preferably not more than about 80 HV 0.05 and desirably not morethan about 50 HV 0.05, and/or at most a 10% higher hardness HV 0.05 thanthe softest mixture in that range and/or a hardness HV 0.05 differing byat most ±10%, preferably at most ±8% and desirably at most ±5% comparedto the mixture of 30 wt. % molybdenum or molybdenum alloy with up to 40wt. % of other constituents/70 wt. % of the steel.

With the hardness criteria set forth above, very good adhesion to thesubstrate (in particular an aluminum alloy) and good workability areachieved in addition to low porosity. If the steel is too hard, thedanger of layer eruptions increases. If the steel is too soft, it willsmear during machining.

Other advantageous criteria for production of a coating according to theinvention are the application parameters and the substrate to which thecoating is applied. Thus, according to the invention, it has been foundthat with molybdenum-steel mixtures, thin layers, i.e. those from 80 to350 microns and preferably 100 to 300 microns, are especially suitable.In the above-mentioned prior art, the corresponding coatings areconsiderably thicker, i.e. in particular 500 microns to 2.5 mm, coatingson the order of 1000 microns being conventionally formed for cylinderlinings. With the use of molybdenum-steel, however, it has been foundaccording to the invention that the thick coatings not only areuneconomical but also have other important disadvantages. Thus, coatingsover 350 microns thick readily break away from the substrate, especiallywhen an aluminum alloy is used as the substrate. The use of thinnerlayers also is not very suitable, since they interfere with thesubsequent mechanical treatment, i.e. honing.

Pretreatment of the substrate surface to be coated is especiallyadvantageous, roughening preferably being achieved not by grinding butby application of a particle or fluid stream. Roughening of the surfaceprior to plasma coating is known in principle, but, for coatingscontaining molybdenum, as described for example in U.S. Pat. No.2,588,422, there has heretofore expressly been no roughening but at mosta cleaning of the surface. According to the invention, however, it hasbeen found that a very good surface adhesion of the molybdenum-steellayer is not achieved without the special roughening process. Especiallyadvantageous in this case is the use of corundum for roughening, whichprovides very good long-term results in comparison with aluminum oxide.

Furthermore, it has been found according to the invention that onlycertain definite mean roughness indexes are suitable and provide layerswith especially good adhesion. Thus it has been found that meanrugosities from about 4 to about 35 microns, preferably about 5 to about25 microns and especially about 5 to about 15 microns, are especiallysuitable for molybdenum-steel coatings. Only moderate layer adhesion isobtained with lower rugosities and an unfavorable coating surfaceresults from higher rugosities.

Other advantageous spray parameters according to the invention are theporosity and the oxide content of the coating, the production ofpredominantly closed isolated pores and an advantageous range of porediameters. According to U.S. Pat. No. 3,620,137 porosities ranging frompractically pore-free to about 15% are possible and porosities of about15% have been obtained by spraying molybdenous coatings on graycastings. According to the invention, however, it has been ascertainedthat when using such high porosities, layer separation may occur uponsubsequent honing especially on aluminum substrates. On the other hand,it is also of no advantage to produce a porosity-free coating. Aporosity of at least about 0.3 vol. % and not more than about 10 vol. %,preferably not more than 7 vol. % has proved advantageous to build up apressure microchamber system on the slide surface. In principle, it isalso advantageous if the pores are predominantly closed, i.e., isolated,rather than connected to each other, and lie within a definite sizerange for example, from about 2 microns to about 40 microns, preferablyabout 3 microns to about 20 microns. This requirement alsodifferentiates the invention from the disclosure of U.S. Pat. No.3,620,137 in which the pores are open so as to communicate with eachother because of their high volume ratio and the majority of the poreshave sizes in a range from 0.1 to 2 microns. Such pores have been foundto be not very suitable for the construction of a pressure microchambersystem with molybdenum-steel coatings according to the invention.

Last of all, steels satisfying at least one of the followingrequirements for the chemical compositions of the steel used in applyingmolybdenum-steel coatings to aluminum have proved favorable:

    ______________________________________                                        C content    0.7-2.1 wt. %,                                                                             esp. 0.8 to 1.5 wt. %                               Cr content   1.0-15 wt. %,                                                                              esp. 1.0 to 5 wt. %                                 Si content   0.5-2 wt. %,                                                     Mo content   2.5-50 wt. %,                                                                              esp. 3 to 15 wt. %                                  W content    2.5-30 wt. %,                                                                              esp. 3 to 15 wt. %                                  Mo + W content                                                                             2.5-50 wt. %,                                                                              esp. 3 to 25 wt. %                                  ______________________________________                                    

α-Fe matrix at least 50 wt. %, especially at least 70 wt. % of Fe and/orfine Cr, Mo, W and/or Mn carbides (not over 1 micron) in the α-Fematrix.

According to the invention, cast iron is not well suited, i.e.iron-carbon alloys with a carbon content from 2 to 4 percent. Bettersuited are steels with less than 2. 1%, which are forgeable.

In principle, coatings of steel in combination with molybdenum are knownin the British Published Application No. 2,050,434, but in that casesuch a mixture produces very poor results on iron substrates.Surprisingly, coatings having such steels are especially advantageous onaluminum alloys. Further, the steels employed according to the inventionhave a chromium content which is advantageously not above 15%. However,a minimal chromium content is especially favorable, since it achieves asuitable hardness of the steel for spraying together with molybdenum.The chromium content is limited essentially by the formation ofaustenite, which renders the steel less suitable for use according tothe invention. The use of silicon is also favorable according to theinvention but small amounts are sufficient. Silicon lowers the meltingpoint of the steel advantageously, so that it can be plasma sprayedespecially well together with the molybdenum. However, silicon valuesabove about 2 wt. % lead to embrittlement of the steel. The use ofmolybdenum as an alloy constituent of the steel, as well as the use oftungsten, is favorable, the upper limit being determined essentially bythe resulting increase in the hardness of the steel and the cost of thealloy constituent. Basically, these elements are contained as alloyconstituents in a definite percentage. Also according to the invention,a steel having an α-Fe matrix in which fine carbides are contained inthe submicroscopic range is advantageous. Such steels are especiallysuitable for spraying with molybdenum. Preferably, in addition tocarbon, the steel used in the coating contains at least one anddesirably at least two of the following elements in order of preference:Cr, Si, Mo, W, Mn and B.

Certain alloy constituents are preferably not present or present only insmall amounts in the steel, for example, boron up to 1 wt. %, nickel upto 0.5 wt. %, desirably up to 0.3 wt. %, manganese up to 4 wt. % andphosphorus up to 0.5 wt. %, desirably up to 0.2 wt. % and sulfur up to0.5 wt. %, desirably up to 0.2 wt. %. Manganese is a suitable carbidestabilizer and boron a carbide former as well, while boron in largeramounts leads to a hardening and embrittlement of the steel.Nevertheless, small amounts may be appropriate to lower the meltingpoint of the steel. Nickel as a ductile material, is preferably notpresent at all, even though, in principle, nickel is regarded as afavorable alloy constituent in steel. The content of phosphorus andsulfur also is preferably kept low to avoid embrittlement of the steel.

In accordance with the invention, slide layers may be bound with specialstrength to the walls of cylinder bores in a light metal engine block ofan internal combustion engine and in an environmentally sound manner.The body to be coated can be an aluminum alloy. The body to be coatedcan be an AlSi alloy. These slide layers may be applied in one coatingstep and require only a little further treatment such as honing and nogalvanic or environmentally questionable processes are employed.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying drawings in which:

FIG. 1 is a magnified cross-sectional view showing schematically thestructure of a representative embodiment of a plasma spray layer appliedto a light metal alloy;

FIG. 2 is a perspective view illustrating schematically a slide surfacehaving a communicating system with limited hydrodynamic lubricationaccording to the prior art;

FIG. 3 is a schematic perspective view similar to FIG. 2 illustrating arepresentative embodiment of a slide surface having pressuremicrochamber system according to the invention;

FIG. 4 is a chart showing the adhesion strength of a plasma layer asfunction of mean rugosity;

FIG. 5 is a bar graph showing layer hardness as a function ofcomposition;

FIG. 6 is a graphical illustration showing reduction in score depth withrespect to molybdenum content; and

FIG. 7 is a graphical illustration showing the bearing ratio at 1 microndepth with respect to molybdenum content.

DESCRIPTION OF PREFERRED EMBODIMENTS

The magnified cross-sectional view of FIG. 1 shows a portion of a metalpart 1 on which a slide surface 2 affording hydrodynamic lubrication isto be produced, in particular a cylinder liner for a cylinder in theengine block of an internal combustion engine. The engine block 1 isproduced from a light metal material such as AlSi₆ Cu₄, in this instanceby pressure casting.

To produce a slide layer suitable to the operating conditions of thecylinder liners, a wear layer 3 is applied to the part 1 by plasmaspraying.

FIG. 1 shows the applied wear layer 3 on the part 1 at highmagnification and schematically. As shown in FIG. 1, the wear layer 3 isbound to the part 1 by mechanical interlocking which is produced becausethe wear layer in the liquid state penetrates the unevenness andundercuts in the surface of the part. In these places positivegeometrical connections are formed upon solidification of the wearlayer. In addition, shrink stresses build up inside the wear layer 3,leading to positive forced engagement between the wear layer 3 and thesurface irregularities of the Part 1.

The wear layer 3 contains inclusions such as impurities 4 and unmeltedspray particles 5 and also has comparatively thin oxide layers 6.

The spray powders used to produce the wear layer 3 contain a certainvolume percentage of a molybdenum powder while the remainder is composedof a powdered steel. The powdered steel may, for example, consist ofiron, molybdenum, chromium, nickel, silicon and boron which, whencombined, provide a very hard steel powder. Alternatively, a tool steelalloy consisting or iron, molybdenum, tungsten and chromium may beemployed or a low alloy steel, based for example on iron, chromium,manganese and carbon, may be used.

The wear layer may be made by spraying a powder consisting of 20-60% byweight of powdered molybdenum with the balance powdered steel.Especially good results have been obtained using 30-50% of powderedmolybdenum and 70-50% of a tool steel alloy powder. A possible optimum,depending among other things on the honing and the depth of theroughness to be produced, is a 50%--50% powder combination.

After solidification of the wear layer 3, machining in the form ofhoning, as disclosed for example in German Offenlegungsschrift No. 44 40713, reduces the wear layer 3 to a desired final thickness.Advantageously, in this honing operation, the remnants of inclusionssuch as individual solidified metal droplets, exposed by abrasion, willbe extracted from the surface, exposing numerous lubrication pockets 7in the form of minute isolated cavities as shown in FIG. 3.

Advantageously, the totality of these isolated lubrication pockets 7forms a pressure microchamber system on the slide surface 2 withplateaus 8 of extremely small rugosity separating the lubricationpockets 7. No further steps, such as the use of fluid jets to exposelubrication pockets healed by machining as in the prior art, arerequired to produce the microchamber system.

FIG. 3 shows a portion of a cylinder lining of a part 1 having a slidesurface 2 and a pressure microchamber system formed by lubricationpockets 7 separated by plateaus 8. Schematically, a piston ring segment9 is shown engaging the slide surface 2 and moving relative to the slidesurface 2 with a direction of motion 10 as in operation of the engine.The lubricant collected in the separated lubrication pockets 7 providesa hydrodynamic lubrication by flotation of the piston ring segment 9.

In contrast to the slide surface having a microchamber system withisolated pockets as shown in FIG. 3, FIG. 2 shows a slide surface withan open, communicating groove system according to the prior art. In thissurface intersecting scores 11, produced by honing, contain lubricantwhich is merely pushed along ahead of the piston ring in the directionof the arrows by the motion of the piston ring segment 9. With thissystem, no hydrodynamic pressure can be built up and, at the edges ofthe scores 11, mixed friction between the piston ring and the cylinderlining is possible.

With suitable adjustment of the process parameters, in particular of thecomposition of the wear layer 3 and of the material removed by honing,it is possible to produce a pressure microchamber system affordinghydrodynamic lubrication by the method of the invention in acomparatively few and simple steps. The composition of the wear layerhas a direct relation to the hardness of the resulting slide surface andthe removal of particles by honing may, for example, be increased byincreasing the proportion of powdered molybdenum.

The specific rate of removal by honing, that is, the ratio of measuredremoval during honing to the time required therefor, was found to behigh enough in the case of all sprayed layers to permit completion ofthe honing operation within preassigned time cycles for the fabricationof an engine block having lined cylinders.

For the molybdenum component, a molybdenum essentially as described inU.S. Pat. No. 2,588,422, i.e. a molybdenum alloy with up to about 40%other constituents, may be used. Preferably, however, a molybdenum isused which has at least 90% by weight of molybdenum and, in particular,a molybdenum having other constituents which do not individually exceed1% is preferred, the molybdenum content preferably being above 95%. Themolybdenum may, for example, have an angular shape.

The steel preferably has a spherical morphology, such as is obtained bynozzle spraying. As the steel component, preferably an alloy steel suchas alloy 101, with about 4% Cr, 5% Mo, 1% Si, 0.9% C and 6% W and thebalance Fe is employed, rather than a high-alloy steel such as alloy 102a very low alloy steel such as alloy 103. The high-alloy steel 102contains about 5% Cr, over 2.1% C, 2% Si, 30% Mo and 0.5% B, while thelow-alloy steel 103 contains about 1.5% Cr, 1.1% C and 1.3% Mn.

With the especially suitable steel alloy 101, the influence of the meanrugosity Ra on the adhesion strength of the coating was tested, using a50/50 mixture with molybdenum at least 98% pure. The result isillustrated in FIG. 4, which shows that, above a rugosity Ra of at least4 microns, especially high adhesion strength of the coating on thealuminum substrate is obtained, whereas below that value, there is abroad scatter towards lower adhesion strengths.

Next, comparative hardness measurements were carried out, the hardnesstests being performed directly on the honed spray layer surfaces with atest load of HV 0.05 using the Vickers small load hardness test. Thetest surfaces were subjected to the load for 10 seconds in eachinstance. Plateaus of adequate size were sought for the test, so thatthe results would not be falsified by pores. Because of the low testload, the hardnesses are only of comparative significance, since a DINStandard measurement of hardness would not be possible because of theporosity and extreme thinness of the layer. For each surface tested, 5single measurements were performed, and their mean was calculated. Itmay be seen in FIG. 5 that the Mo-steel 102 plasma spray layers were thehardest and great fluctuations in hardness appeared with increasingmolybdenum content. It turned out that, with the best steel according tothe invention, alloy 101, only slight fluctuations in hardness areobtained, whereas with the low alloy steel 103, the lowest hardnessesare obtained, again with marked fluctuations of hardness as a functionof the proportion of molybdenum. Steels having a low hardness variationas a function of molybdenum content in the spray power are the bestsuited for coating cylinder linings in aluminum engine blocks. Inaddition, coatings in an intermediate hardness range, about HV 0.05, areespecially suitable.

The honed plasma spray layers were also tested for their rugosityprofiles, from which conclusions concerning honability and suitabilityfor use in internal combustion engines may be drawn. By way of example,the reduced scoring depth (Rvk) and the bearing ratios tp⁻¹ % againstmolybdenum content are reproduced in FIGS. 6 and 7, respectively. Theroughness measurements were performed with a Perthometer S8P 5.6. Foreach of three internally coated cylinders, three measurements weretaken. Especially favorable values of reduced score depth are at one totwo microns, which are attained without difficulty with the 101 steel.The bearing ratios serve to support the piston and the piston rings, andshould be as large as possible. Here again, the best measurements wereobtained with the 101 steel.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention.

We claim:
 1. A method for producing a slide surface on a body comprisingthermal spraying a powder mixture of (a) 10 to 70 wt. % molybdenum ormolybdenum alloy with up to about 40 wt. % of other constituents, (b) 90to 30 wt. % of a steel having an α-Fe matrix of at least 50 wt. % and/orfrom 0.8 to 2.1 wt. % carbon and/or from 1.0 to 15 wt. % chromium, (c)and from 0 to 50 wt. % of one or more other components on the body toform a coating providing a slide surface on the body, wherein the steelis selected from the group consisting of: a steel having a hardness from300 to 500 HV 0.3 after thermal spraying; a steel (annealed) having ahardness from 300 to 450 HV 0.3 before thermal spraying; a steel havinga hardness selected so that a thermally sprayed mixture of 30 wt. %molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents and 70 wt. % of the steel has a hardness from 550 to 850 HV0.05; a steel having a hardness selected so that a thermally sprayedmixture of 50 wt. % molybdenum or molybdenum alloy with up to about 40wt. % of other constituents and 50 wt. % of the steel has a hardnessfrom 550 to 850 HV 0.05; and a steel having a hardness selected so thatthermally sprayed mixtures in the range from about 30 wt. % molybdenumor molybdenum alloy with up to about 40 wt. % of other constituents/70wt. % of the steel to about 50 wt. % molybdenum or molybdenum alloy withup to about 40 wt. % of other constituents/50 wt. % of the steel have ahardness difference selected from the group consisting of: (i) at most100 HV 0.05, (ii) at most a 10% higher hardness HV 0.05 than the softestmixture in that range and (iii) a hardness HV 0.05 differing by at most±10% compared to a mixture of 30 wt. % molybdenum or molybdenum alloywith up to about 40 wt. % of other constituents/70 wt. % of the steel.2. A method according to claim 1, wherein the hardness of the sprayedsteel is in the range from about 350 to about 450 HV 0.3.
 3. A methodaccording to claim 1 wherein the hardness of a thermally sprayed mixtureof 30 wt. % molybdenum or molybdenum alloy with up to about 40 wt. % ofother constituents/70 wt. % steel is in the range from about 600 toabout 750 HV 0.05.
 4. A method according to claim 3 wherein the hardnessof a thermally sprayed mixture of 30 wt. % molybdenum or molybdenumalloy with up to about 40 wt. % of other constituents/70 wt. % steel isin the range from about 650 to about 700 HV 0.05.
 5. A method accordingto claim 1 wherein the hardness of a thermally sprayed mixture of 50 wt.% molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents/50 wt. % steel is in the range from about 600 to 750 HV0.05.
 6. A method according to claim 5 wherein the hardness of athermally sprayed mixture of 50 wt. % molybdenum or molybdenum alloywith up to about 40 wt. % of other constituents/50 wt. % steel is in therange from about 650 to 700 HV 0.05.
 7. A method according to claim 1wherein the steel has a hardness such that thermally sprayed mixtures inthe range from 30 wt. % molybdenum or molybdenum alloy with up to about40 wt. % of other constituents/70 wt. % of the steel to 50 wt. %molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents/50 wt. % of the steel have a hardness in the hardness rangefrom about 550 to about 850 HV 0.05.
 8. A method according to claim 7wherein the hardness of the steel is such that the thermally sprayedmixtures have a hardness in the range from about 600 to about 750 HV0.05.
 9. A method according to claim 8 wherein the hardness of the steelis such that the thermally sprayed mixtures have a hardness in the rangefrom about 650 to about 700 HV 0.05.
 10. A method according to claim 1wherein thermally sprayed mixtures in the range from 30 wt. % molybdenumor molybdenum alloy with up to about 40 wt. % of other constituents/70wt. % of the steel to 60 wt. % molybdenum or molybdenum alloy with up toabout 40 wt. % of other constituents/40 wt. % of the steel satisfies oneof the specified hardness difference requirements.
 11. A methodaccording to claim 1 wherein thermally sprayed mixtures in the rangefrom about 30 wt. % molybdenum or molybdenum alloy with up to about 40wt. % of other constituents/70 wt. % of the steel to about 50 wt. %molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents/50 wt. % of the steel have a hardness difference of notmore than about 80 HV 0.05.
 12. A method according to claim 11 whereinthe thermally sprayed mixtures have a hardness difference of not morethan about 50 HV 0.05.
 13. A method according to claim 1 whereinthermally sprayed mixtures in the range from about 30 wt. % molybdenumor molybdenum alloy with up to about 40 wt. % of other constituents/70wt. % of the steel to about 50 wt. % molybdenum or molybdenum alloy withup to about 40 wt. % of other constituents/50 wt. % of the steel have atmost an 8% higher hardness HV 0.05 than the softest mixture in thatrange.
 14. A method according to claim 13 wherein thermally sprayedmixtures in the range from about 30 wt. % molybdenum or molybdenum alloywith up to about 40 wt. % of other constituents/70 wt. % of the steel toabout 50 wt. % molybdenum or molybdenum alloy with up to about 40 wt. %of other constituents/50 wt. % of the steel have at most a 5% higherhardness HV 0.05 than the softest mixture in that range.
 15. A methodaccording to claim 1 wherein thermally sprayed mixtures in the rangefrom about 30 wt. % molybdenum or molybdenum alloy with up to about 40wt. % of other constituents/70 wt. % of the steel to about 50 wt. %molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents/50 wt. % of the steel have a hardness HV 0.05 deviating byat most ±5% referred with regard to that of the mixture of 30 wt. %molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents /70 wt. % of the steel.
 16. A method according to claim 1wherein the body to which the coating is applied is an aluminum alloy.17. A method according to claim 16 wherein the body is an AlSi alloy.18. A method according to claim 1 wherein the steel satisfies at leastone requirement selected from the group consisting of:

    ______________________________________                                        C content            0.7-2.1 wt. %;                                           Cr content           1.0-15 wt. %;                                            Si content           0.5-2 wt. %;                                             Mo content           2.6-50 wt. %;                                            W content            2.5-30 wt. %;                                            Mo + W content       2.5-50 wt. %;                                            ______________________________________                                    

α-Fe matrix at least 50 wt. % of Fe; fine Cr, Mo, W and/or Mn carbides(not over 1 micron) in an α-Fe matrix;

    ______________________________________                                        B content          up to 1 wt. %;                                             Ni content         up to 0.5 wt. %;                                           Mn content         up to 4 wt. %;                                             P content          up to 0.5 wt. %; and                                       S content          up to 0.5 wt. %.                                           ______________________________________                                    


19. A method according to claim 18 wherein the steel satisfies at leastone2requirement from the group consisting of:

    ______________________________________                                        C content           0.8 to 1.5 wt. %;                                         Cr content          1.0 to 5;                                                 Mo content          3 to 15;                                                  W content           3 to 15;                                                  Mo + W content      3 to 25;                                                  ______________________________________                                    

α-Fe matrix at least 70 wt. % of Fe;

    ______________________________________                                        Ni content        up to 0.3 wt. %;                                            P content         up to 0.2 wt. %; and                                        S content         up to 0.2 wt. %.                                            ______________________________________                                    


20. A method according to claim 1 wherein the steel also satisfies atleast one requirement selected from the group consisting of:

    ______________________________________                                        B content          up to 1 wt. %;                                             Ni content         up to 0.5 wt. %;                                           Mn content         up to 4 wt. %;                                             P content          up to 0.5 wt. %; and                                       S content          up to 0.5 wt. %.                                           ______________________________________                                    


21. A method according to claim 20 wherein the steel also satisfies atleast one requirement selected from the group consisting of:

    ______________________________________                                        Ni content         up to 0.3 wt. %;                                           P content          up to 0.2 wt. %; and                                       S content          up to 0.2 wt. %.                                           ______________________________________                                    


22. A method according to claim 1 wherein the body is a cylinder liningsurface of an internal combustion engine.
 23. A method according toclaim 1 wherein the body is an aluminum engine block.
 24. A methodaccording to claim 1 including removing part of the coating by amechanical process.
 25. A method according to claim 1 wherein theapplication of the mixture to form a coating satisfies at least one ofthe requirements selected from the group consisting of:a coatingthickness from 80 to 350 microns; the coating is applied to a surface ofthe body which has been roughened by a flow of a stream of particlesand/or fluid; the coating is applied to a surface having a mean rugosity(Ra) from about 4 to about 35 microns; the coating has a porosity of notmore than about 10 vol. %; the coating has predominantly isolated pores;and the coating has a majority of pores in a size range from about 2 toabout 40 microns.
 26. A method for producing a slide surface on a bodycomprising thermal spraying a powder mixture of (a) 10 to 70 wt. %molybdenum or molybdenum alloy with up to about 40 wt. % of otherconstituents and (b) 90 to 30 wt. % of a steel and (c) from 0 to 50 wt.% of one or more other components to form a coating providing a slidesurface on the body, wherein the coating is applied so as to satisfy atleast one characteristic selected from the group consisting of:toprovide a coating with a thickness from 80 to 350 microns; applicationto a surface having a mean rugosity (Ra) in the range from about 4 toabout 35 microns; to provide a coating with a porosity not more thanabout 10 vol. %; to provide a coating with predominantly isolated pores;to provide a coating with a majority of the pores in a size range fromabout 2 to about 40 microns.
 27. A method according to claim 25 or claim26 wherein the thickness of the layer is 100 to 300 microns.
 28. Amethod according to claim 25 or claim 26 wherein the porosity of thecoating is no more than about 7 vol. %.
 29. A method according to claim25 or claim 26 wherein the majority of the pores size in the range fromabout 3 to about 20 microns.
 30. A method according to claim 25 or claim26 wherein the surface of the body is pretreated with corundum toroughen the surface.
 31. A method according to claim 25 or claim 26wherein the surface of the body has a mean rugosity (Ra) in the rangefrom about 5 to about 25 microns.
 32. A method according to claim 31wherein the surface of the body has a mean rugosity (Ra) in the rangefrom about 5 to about 15 microns.
 33. A method for producing a slidesurface on an aluminum alloy comprising thermal spraying a powdermixture of (a) 10 to 70 wt. % molybdenum or molybdenum alloy with up toabout 40 wt. % of other constituents and (b) 90 to 30 wt. % of a steelto form a coating having a slide surface on the aluminum alloy whereinthe steel satisfies at least one requirement selected from the groupconsisting of:

    ______________________________________                                        C content            0.7-2.1 wt. %;                                           Cr content           1.0-15 wt. %;                                            Si content           0.5-2 wt. %;                                             Mo content           2.5-50 wt. %;                                            W content            2.5-30 wt. %;                                            Mo + W content       2.5-50 wt. %;                                            ______________________________________                                    

α-Fe matrix at least 50 wt. %; and fine Cr, Mo, W and/or Mn carbides(not over 1 micron) in an α-Fe matrix.
 34. A method according to claim33 wherein the steel satisfies at least one requirement selected fromthe group consisting of:

    ______________________________________                                        C content           0.8 to 1.5 wt. %;                                         Cr content          1.0 to 5;                                                 Mo content          3 to 15;                                                  W content           3 to 15;                                                  Mo + W content      3 to 25; and                                              ______________________________________                                    

α-Fe matrix at least 70 wt. % of Fe.